The term "laser" is an acronym for light amplification by stimulated emission. Stimulated emission occurs when an electromagnetic wave encounters atoms in material at high energy level E.sub.2 and force the atom to a lower energy level E.sub.1. The atoms undergoing this transition emit an electromagnetic wave in the process. The emitted wave adds to the incident wave and travels in the same direction. The process by which the atoms in the material are raised from a lower energy level E.sub.1 to a higher level E.sub.2, in order to be stimulated to emission, is called the pumping process.
Laser material is generally pumped either optically or electrically. The present invention is concerned with optical or electrical pumping of semiconductor lasers, i.e., lasers in which the active region of the laser consists of semiconductor (GaAs, GaAlAs, etc.) material.
Semiconductor lasers generally comprise a length of active laser material sandwiched between upper and lower cladding material.
Generally, optical pumping of solid state lasers is accomplished by directing a beam of light from a laser diode at the longitudinal edge of the laser material. Electrical pumping occurs by application of a voltage across a p/n junction of light emitting material to inject electrons and holes and obtain population inversion.
Lasers may be formed with either an internal cavity or an external cavity. An internal cavity laser is formed by cleaving the longitudinal ends of the laser material and forming optical mirror surfaces at each end, so light is reflected back and forth through the active material and eventually emitted out one end or facet of the laser.
In an external cavity laser structure, a first facet is coated with a multilayer, high reflection coating and the opposite second facet is coated with an antireflection (AR) coating. A lens is positioned to collimate light emitted from the AR coated facet. The collimated light passes to a mirror which is preferably located at the focal point of the lens. Optical feedback from the mirror to the laser is used to control the frequency and modality of the laser light emitted out the first facet.
This present invention relates to external cavity semiconductor laser systems.